Follow-up from "Use a serializer to render diff lines"
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Refactor `Projects::MergeRequests::ConflictsController` to use `DiffFileEntity` when rendering json if possible. If it is not possible then create a new entity/serializer for `Conflict::File`.
The following discussion from gitlab-ce!21260 should be addressed:
- [ ] @felipe_artur started a [discussion](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21260#note_95177227): (+1 comment)
> We should open another gitlab-ce~3011693 to cover this.`Projects::MergeRequests::ConflictsController` is rendering diff file in a different way and not using the default serializer.
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